Conventional magnetic RAM (Random Access Memory) stores data via magnetic memory elements as opposed to electric charge or current flows. Conventional magnetic memory elements are formed from multiple ferromagnetic layers. Each of the layers has a magnetization vector that can point in one of several directions, storing a magnetization state. A thin non-magnetic layer (e.g., a thin insulator layer or metal) typically separates the layers. If the non-magnetic layer is a metal the structure is known as a spin valve. It the non-magnetic layer is a thin insulating layer the structure is known as a magnetic tunnel junction. One of the two layers can be a permanent magnet set to a particular polarity; a field of the other layer can be changed to store data.
This configuration is the simplest structure for a MRAM bit. A typical magnetic memory device includes an array of memory “cells”. Via an MRAM, a user can store substantial amounts of data. In a conventional MRAM cell magnetic fields generated by current carrying wires (i.e. an Amperian field) near the cells are used to reorient the magnetic field (or magnetization) in one of the layers to store data. This also known as field switched MRAM.
According to a newer conventional technique, Spin Transfer Torque (STT) or spin transfer switching, uses spin-aligned (“polarized”) electrons to apply a torque directly to the magnetization of the layers. For example, if the electrons flowing into a layer have to change their spin direction, this develops a torque that will be transferred to the nearby layer. This significantly reduces an amount of current needed to write the cells from that of a conventional magnetic field switched MRAM cell.
There are concerns that the conventional type of MRAM cell will have difficulty at high cell densities due to the amount of current needed during writes, a problem that STT overcomes. For this reason, the STT proponents expect the technique to be used for smaller sized devices. For example, spin coherence may not be needed. The device needs to be smaller than the spin-diffusion length, which is around 100 nanometers or larger at low temperatures and which is easy to achieve. Overall, the STT requires much less write current than conventional or toggle MRAM. Research in this field indicates that STT current can be reduced by orders of magnitude by using a new composite structure. However, higher speed operation typically requires higher current.
So-called spin-torque transfer-RAM (STT-RAM) encodes non-volatile information in the relative alignment of the magnetizations of two magnetic layers—one fixed and one free. Magnetoresistance is a measure of the resistance of the magnetic device. A magnitude of the magnetoresistance depends on the relative alignment of the magnetization of two or more layers. One then can read the device by determining its resistance. As mentioned above, this is done in present day field switched MRAMs. STT-RAM differs from conventional MRAM by utilizing the torque exerted by an ensemble of spin-polarized electrons (or holes) to effect a rotation of the free layer through a short-range exchange interaction.
This conventional approach has allowed scaling of device size to much smaller dimensions than possible by the use of Amperian-field driven switching found in conventional MRAM. The spin-polarized current derives from all the magnetic layers in a device. The key point is that when one has magnetic layers and the current passes through these magnetic layers, the current acquires a spin-polarization.